When detecting flaws of metal materials such as pipelines by using the remote field eddy current method, a remote field eddy current sensor connected to a cable for transmitting signals is inserted into the pipeline, and exciting voltage is supplied to the sensor. The sensor includes an exciting coil and one or more receiving coils spaced from the exciting coil in the longitudinal direction of the pipeline at a predetermined distance being two times longer than a diameter of the pipeline. A relatively low frequency voltage having a frequency such as from tens Hz to hundreds Hz and a voltage from several V to tens V is used as the exciting voltage signal.
Electromagnetic waves generated by the exciting signal are separated into two groups; one passes through a thickness of the pipeline to be tested and the other propagates in the pipeline. The latter is rapidly attenuated and hardly transmitted, since, if the pipeline is considered to be a wave guide, its frequency is much lower than the cut-off frequency. The former wave is called an indirect transmitting wave and transmits along the exterior of the pipeline and is gradually attenuated. At the same time, a part of the former wave passes again through the thickness of the pipeline, permeates into the pipeline and is received by the receiving coil.
The received signal detected by the receiving coil has very small magnitude (from several micro V to tens micro V), since it passes through the pipeline twice. In addition, the phase thereof is changed by the skin effect in passing through the thickness of the pipeline. In the remote field eddy current method, that phase change has good linearity relative to the thickness of the pipeline and therefore may be used as an indication of pipe thickness and the presence of flawed pipe walls (i.e. areas of nonuniform thickness).
In the remote field eddy current method, when the eddy current sensor including the exciting coil and the receiving coil is inserted into the pipeline to be tested and progressed at a constant velocity, the amplitude of the received signal is changed and phase detected flaw data is mixed with phase detection noise, since the dielectric constant of the pipeline to be tested is not uniform and the eddy current sensor vibrates as a result of the progressing movement. Thus, the detected flaw data may be converted abnormally. Further, when another type of the eddy current sensor is used and wherein a plurality of the receiving coil is arranged annularly in the inner wall of the pipeline, it is difficult to make accurate diagnosis of the pipeline, since abnormal flaw data is accumulated.
The remote field eddy current method can be classified into two types; an absolute type in which a plurality of receiving coils are arranged concentrically at the rear of an exciting coil and a differential type which includes a front group of the receiving coils concentrically arranged and a rear group of the receiving coils arranged at the rear of the front group. In the absolute type, the number of turns of each coil of the group of the receiving coils is the same, respectively, and a plurality of receiving coils is connected with a measuring device by a required number of pair cables for producing a sensor signal. On the other hand, in the differential type, the number of turns of the front group receiving coils and the rear group receiving coils is the same, respectively, and the front coil and the rear coil are differentially connected to each other and are connected with a measuring device by a required number of pair cables.
While in the absolute type sensor the received signal can be always received in a part of the pipeline without flaws, in the differential type sensor, the received signal is usually received with a very small magnitude except in a part of the pipe having a changed shape such as a local flaw, because of the differential connection. This phenomenon also is found when using another type of receiving coil (normal direction coil) in which magnetic path by the remote field eddy current is arranged normal to the axis of the exciting coil.
When using the group of receiving coils including the differential type coil or the normal direction coil in the remote field eddy current method in a normal part of material to be tested, it is difficult to get sufficient received signal in order to obtain a stable measurement of phase. In addition, phase detected flaw data is often mixed with noise, and the flaw data may be converted erroneously.
Further, when using the remote field eddy current sensor in which a plurality of the receiving coils are arranged annularly in the inner wall of the pipeline, it is difficult to make accurate diagnosis of the pipeline because of the accumulation of erroneous flaw data.
In the differential type sensor, the number of turns of each receiving coil of the front group and the rear group of the receiving coils is the same, respectively, and required number of coils is parallel or serially connected in the front group or the rear group. The front coils and the rear coils are differentially connected to each other and connected with the measuring device by the required number of pair cables.
In the absolute type of the remote field eddy current sensor having the above mentioned construction, while stable diagnosis can be made because relatively high level sensor signal can be provided and the sensor is suitable for the detection of gradually progressing flaws in part FW of the pipeline to be tested, the detecting sensitivity to a locally flawed part of the pipeline FS is low and frequently the sensor cannot detect such small local flaws. On the other hand, in the differential type of the remote field eddy current sensor, while the flawed part of the pipe can be detected by the signal level difference between the front group of the receiving coils and the rear group of the receiving coils, the differential sensor can detect local flaws because of its high sensitivity. However, it is hard to measure the difference generated for gradually progressing flaws. As a result, it is hard to make stable phase detection because the differential signal is too small.
In both types of remote field eddy current sensors, since sensor measurement is performed not only by strength and weakness of the sensor signal level, but also by computing width or depth of the flaw by phase delay characteristics of an acquired sensor signal, it is important to get stability of the phase detection.